In order to exchange information between transmitting and receiving devices, information is typically upconverted from baseband frequencies (i.e., very low to DC frequencies) to a carrier frequency. The upconverted information is then transmitted over a communication medium. A receiving device typically processes the received information at baseband frequencies. Accordingly, receiving devices typically employ mixers to downconvert the received information from the received frequencies to baseband frequencies.
FIG. 1 illustrates a conventional I Q receiver mixer, which is commonly referred to as a Gilbert mixer. The mixer of FIG. 1 performs quadrature demodulation down to a zero, or very low intermediate frequency by splitting a received signal into two signals and applying each of the signals to quadrature mixers for the I and Q channels.
A typical Gilbert mixer includes a transconductance stage and a mixer core stage. The transconductance stage includes a pair of transistors, (M1 and M2; or M11 and M12), which have their sources coupled to a current source, Icp, and their gates are coupled to the received signal. Each of the transistors, (M1 and M2; or M11 and M12), have their drains coupled to the mixer core stage. Specifically, the drains of each transistor are coupled to the sources of a pair of transistors (M3 and M4 or M5 and M6 for the I mixer; and M7 and M8 or M9 and M10 for the Q mixer) in the mixer core stage.
The gates of each transistor in the mixer core stage (M3-M6 for the I mixer and M7-M10 for the Q mixer) are coupled to local oscillators, which provide a frequency signal, such as a sine wave. Each transistor of the pair of transistors in the mixer core stage receives a frequency signal that is offset from the other transistor of the pair by 180 degrees. As illustrated in FIG. 1, for the I mixer the sources of M3 and M6 receive a local oscillator signal that has a 180 degree offset from the local oscillator signal received by the sources of M4 and M5. Similarly, for the Q mixer the sources of M7 and M10 receive a local oscillator signal that has a 180 degree offset from the local oscillator signal received by the sources of M8 and M9. In order to form the I and Q signals, the local oscillator frequencies provided to the gates of the transistors of the Q mixer are offset 90 degrees from the local oscillator frequency signals provided to the gates of the transistors of the I mixer.
A differential I output signal is formed by coupling the drain of M4 to the drain of M6, and the drain of M5 to the drain of M3. Similarly, a differential Q output signal is formed by coupling the drain of M8 to the drain of M10, and the drain of M9 to the drain of M7.